1. Field of the Invention
The present invention relates to a wire electric discharge machine and, more particularly, to a wire electric discharge machine capable of reducing a shape error due to bending of a wire electrode on the upper and lower surfaces of workpiece and attaining improvement in shape accuracy of the workpiece by correcting, independently in an upper guide section and a lower guide section, the positions of upper and lower wire guides in a relative movement command of a wire guide with respect to the workpiece commanded by a machining program.
2. Description of the Related Art
In wire electric discharge machining, it is known that a wire electrode bends in the opposite direction of a machining progress direction or a direction at a fixed angle from the machining progress direction because of a discharge repulsion force that occurs between the wire electrode and a workpiece, a turbulent flow of machining liquid, or the like. When workpiece is linearly machined by the wire electrode, although bending of the wire electrode occurs in a direction opposite to the machining progress direction, the bending of the wire electrode does not affect a machining shape. However, large influence of the bending of the wire electrode appears in machining of a corner section machined at a predetermined angle set in a machining program and greatly deteriorates shape accuracy of the corner section. So-called “corner sagging” occurs. An intended machined shape is not obtained.
FIG. 18 is a diagram for explaining machining of a corner section. In the wire electric discharge machining, in order to machine workpiece 3 in dimensions as desired, in general, an offset route is created by adding the radius of a wire electrode 4 and a discharge gap to dimensions of an actual product shape and the wire electrode 4 is moved along the created offset route (a machining route 5). Note that a value obtained by adding up the radius of the wire electrode 4 and the discharge gap is referred to as offset value.
As shown in FIG. 18, since the wire electrode 4 bends in a corner section of the workpiece 3, the wire electrode 4 does not move along the machining route 5 and moves on the inner side of the machining route (see a track 6a of the wire electrode indicated by a broken line). As a result, the wire electrode 4 excessively machines the workpiece 3. Therefore, so-called “corner sagging” (see corner sagging 7 due to bending of the wire electrode) is formed. A desired machining finish shape is not obtained. So, various measures have been proposed to cope with such a problem.
Japanese Patent Application Laid-Open No. 58-120428, Japanese Patent Application Laid-Open No. 8-39356, and Japanese Patent Application Laid-Open No. 7-24645 mention that a shape failure due to corner sagging is reduced by reducing machining energy when a wire electrode passes a corner section, minimizing bending of the wire electrode, and causing the wire electrode to pass the corner section.
In the machining method, the machining energy is reduced, that is, machining speed is reduced until the bending of the wire electrode decreases. The corner section is machined slowly. Therefore, there is a disadvantage that a machining time is long.
Japanese Patent Application Laid-Open No. 8-39356, Japanese Patent Application Laid-Open No. 7-24645, Japanese Patent Application Laid-Open No. 11-221719, and Japanese Patent Application Laid-Open No. 2013-190854 mention that, when a wire electrode passes a corner section, bending of the wire electrode is corrected from a machining route and a so-called go-too-far and return route is given to the wire electrode to correct a shape error due to the bending of the wire electrode. In the machining method, since deceleration is not performed unlike the machining method explained above, a machining time is not excessively long.
As shown in FIG. 19, when the machining route 5 of the wire electrode 4 is corrected by a correction route 8 to perform the wire electric discharge machining, the wire electrode 4 moves along a wire electrode track 6b. Consequently, it is possible to prevent the workpiece 3 from being excessively machined.
FIG. 20 is a flowchart of processing for calculating and outputting a correction route on the basis of the distance between upper and lower wire guides according to the related art. In this processing, correction is not performed independently in the upper and lower wire guides.
Information concerning the thickness of workpiece and an upper wire guide position and a lower wire guide position in a state in which upper and lower nozzles are closely attached to the workpiece is acquired (step SA01). A correction amount C is calculated according to a bending amount E of a wire electrode in the state in which the upper and lower nozzles are closely attached to the workpiece (step SA02). A true correction amount C is calculated from the calculated correction amount C and angle information of a corner section (step SA03). A correction route is calculated on the basis of the calculated true correction amount C (step SA04). The calculated correction route is output (step SA05). Then, this processing is ended.
In the measures of both of Japanese Patent Application Laid-Open No. 7-24645 and Japanese Patent Application Laid-Open No. 11-221719, it is assumed that relative positional relations between the positions of the upper and lower wire guides for supporting the wire electrode and the upper surface and the lower surface of the workpiece (i.e., a gap between the upper guide position and the upper surface of the workpiece and a gap between the lower guide position and the lower surface of the workpiece) are substantially the same distances and wire bending amounts are substantially the same on the upper surface and the lower surface of the workpiece in a state in which the upper and lower nozzles for ejecting the machining liquid are closely attached to the workpiece upper and lower surfaces. It is assumed that correction amounts for shape correction are also the same on the upper surface and the lower surface. That is, the shape correction is performed according to X and Y axes on a two-dimensional plane.
However, machining by the wire electric discharge machine is not always machining in which the guide sections and the nozzles are closely attached to the upper and lower surfaces of the workpiece. For example, in some case, the lower surface of the workpiece is fixed on a fixed table (closely attached to the fixed table) and, on the other hand, the upper guide section is greatly apart from the workpiece upper surface because of a fixing jig or other reasons.
When the upper guide section for supporting the wire electrode is greatly apart from the workpiece upper surface and the lower guide section for supporting the wire electrode is closely attached to the workpiece lower surface, on the workpiece lower surface, a shape correction amount in the corner section may be the correction amount itself, that is, a shape correction block. However, on the workpiece upper surface, bending due to vibration of the wire electrode further increases. Therefore, correction of the workpiece upper surface is not correctly performed even if the shape of the workpiece lower surface is corrected. Consequently, improvement of shape accuracy of a machining shape of the workpiece is insufficient.